The hydrophobic adsorption of charged molecules to bilayer membranes: a test of the applicability of the Stern equation

Abstract

To describe the hydrophobic adsorption of charged molecules to bilayer membranes, that the adsorption produces a change in the electrostatic potential at the surface of the membrane must be recognized. The surface potential produced by the adsorption of the charged molecules can be simply described by the Gouy equation from the theory of the diffuse double layer. This potential tends to lower the concentration of the adsorbing ions in the aqueous phase immediately adjacent to the membrane, a phenomenon which can be described by the Boltzmann relation. The number of adsorbed ions is a function of the aqueous concentration of these ions at the membrane solution interface and can be described, in the simplest case, by a Langmuir adsorption isotherm. If the ions are regarded as point charges, the combination of the Gouy, Boltzmann and Langmuir relations may be considered a simplified Stern equation. To test experimentally the applicability of this equation, both charge density and surface potential as a function of the concentration of adsorbing molecules in the bulk aqueous phases should be measured. Direct, accurate measurements of 1 of these parameters, the number of moles of 2,6-toluidinylnaphthalenesulfonate ions bound to vesicles formed from phosphatidylcholine, are available in the literature (C. Huang, and J.P. Charlton (1972)). The authors estimated the change in the surface potential in 2 independent ways: by conductance measurements with probe molecules on planar black lipid membranes and by electrophoresis measurements on multilaminar unsonicated vesicles. The 2 estimates agreed with one another, and all of the data could be adequately described by the Stern equation, assuming, at 25.degree. C, a dissociation constant of 2 .times. 10-4 M and a maximum number of binding sites of 1/70 .ANG.2. [Bacterial phosphatidylethanolamine was used.].